This section is intended to provide a background or context to the invention that is recited in the claims. The description herein may include concepts that could be pursued, but are not necessarily ones that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, what is described in this section is not prior art to the description and claims in this application and is not admitted to be prior art by inclusion in this section.
A communication system known as evolved UTRAN (EUTRAN, also referred to as UTRAN-LTE for Long Term Evolution or as E-UTRA, see below for acronym meanings) is currently under development within the third generation partnership project (3GPP). As presently specified the downlink (DL, network access node eNB toward user equipment UE) access technique will be orthogonal frequency division multiple access OFDMA, and the uplink (UL, UE toward eNB) access technique will be single carrier frequency division multiple access (SC-FDMA).
FIG. 1 reproduces FIG. 4.1 of 3GPP TS 36.300 V8.6.0 (2008-09), 3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Access Network (E-UTRAN); Overall description; Stage 2 (Release 8). FIG. 1 shows the overall architecture of the E-UTRAN system. The EUTRAN system includes eNBs, providing the EUTRA user plane (packet data convergence protocol PDCP/radio link control RLC/medium access control MAC/physical PHY layers) and control plane (radio resource control layer) protocol terminations towards the UE. The eNBs are interconnected with each other by means of an X2 interface. The eNBs are also connected by means of an S1 interface to an evolved packet core EPC, more specifically to a mobility management entity (MME) by means of a S1 MME interface and to a serving gateway (SGW) by means of a S1 interface. The S1 interface supports a many to many relationship between MMEs/Serving Gateways and eNBs.
The eNB hosts the following functions:                functions for Radio Resource Management: Radio Bearer Control, Radio Admission Control, Connection Mobility Control, Dynamic allocation of resources to UEs in both uplink and downlink (scheduling);        IP header compression and encryption of the user data stream;        selection of a MME at UE attachment;        routing of User Plane data towards Serving Gateway;        scheduling and transmission of paging messages (originated from the MME);        scheduling and transmission of broadcast information (originated from the MME or O&M); and        a measurement and measurement reporting configuration for mobility and scheduling.        
Of particular interest to certain embodiments of the invention detailed below are the further releases of 3GPP LTE targeted towards future International Mobile Telecommunications IMT-A systems, referred to herein for convenience simply as LTE-A (LTE-Advanced). 3GPP TR 36.913, and 3GPP TR 36.814 each pertain to LTE-A and describe it to be an extension over LTE, particularly LTE Release-9. Specifically, LTE-A seeks to improve performance of multi-user (MU) and collaborative (C-) multiple-input/multiple output (MIMO) in order to provide higher data-rates with a limited amount of spectrum, i.e. high spectral efficiency. LTE-A also envisions high-layer transmission (e.g., up to 8). In order to obtain gains from spatial multiplexing methods in LTE-A, it is necessary that the eNB have sufficient knowledge of the wireless channel state information (CSI) that is seen by the UE. In LTE Rel-8, the downlink channel state information is jointly quantized at the UE and fed back in uplink through the physical uplink control channel PUCCH and/or the physical uplink shared channel PUSCH. This is done by mapping the set of n bits of quantization points to a codebook of codewords. The index of a preferred codeword is reported by the UE to the eNB, where ‘preferred’ codeword indicates the codeword that the UE determines would maximize the received power (or otherwise optimize a pre-determined measure) from the eNB on the channel for which the UE just measured CSI if such codeword was applied by the eNB for spatial precoding to transmit to that particular UE.
In order to deliver considerable throughput gains, multi-user MIMO (also known as spatial division multiple access, SDMA) and cooperative MIMO techniques (also referred to as cooperative multipoint transmission, CoMP) require very accurate channel state information at the transmitter. Typically channel state information feedback is based on codebooks as noted above. In some instances the UE feeds back a codeword that represents a quantized version of the wireless channel, which typically the eNB uses the complex conjugate or Hermitian transpose of the reported codeword(s) as transmit precoding weights. In other instances the UE feeds back a codeword that is a precoder recommendation that represents already the preferred precoding weights to be applied at eNB for transmit precoding (i.e. no complex conjugate or Hermitian transpose operation at the eNB is needed).
Limited uplink feedback capacity and UE computational complexity constraints (since the UE has to evaluate the codebook entries in order to select the proper codeword) are the typical limitations leading to the current typical practice of small codebook sizes. For example, the LTE Rel-8 codebook has been designed to be a tradeoff between its size and performance for single-user SU-MIMO. However, what may be sufficient for SU-MIMO is not necessarily sufficient for multi-user MU-MIMO or cooperative MIMO. For example, while SU-MIMO codebook entries provide sufficient orthogonality to be used for multi-layer SU-MIMO transmission, this is not sufficient in the MU-MIMO mode because the degree of inter-user interference is much higher. One limiting factor is the size of the codebook itself.
It appears on the surface that a simple solution would be to increase the size of the codebook. However with increasing codebook size, the complexity of codeword selection at the UE grows, exponentially in fact. Moreover, the standardization of a large codebook would be an issue, and furthermore larger codebook size tends to increase the required signaling overhead.
Of the various prior art approaches of which the inventors are aware that seek to limit codebook size while providing accurate enough channel state information, most are seen to require some expansion of the codebook size and therefore added processing at the UE terminal as well as increased reporting overhead in the uplink direction. As noted above, larger codebooks also become increasingly difficult to standardize.
One specific prior art approach is detailed at Tdoc R1-091288; 3GPP TSG RAN WG1 Meeting #56bis (Seoul, Korea, 23-27 Mar. 2009) by Philips and entitled “CSI FEEDBACK IMPROVEMENTS FOR LTE-A BASED ON MULTIPLE CODEBOOKS”, which its title makes clear proposes multiple codebooks: a coarse codebook of dimensions M and a refinement codebook of dimensions M−1, where there are M transmit antennas at the eNB. In short, that paper describes that the refinement codebook is a projection of the coarse codebook to an orthogonal space of a previously reported codeword taken from the coarse codebook. That paper does not resolve two issues. First, the increased complexity at the UE terminal, which after constructing the refinement codebook must search again for the best matching codeword. Second, it appears that the refinement codebook has to be computed on the fly (which is a computational burden) or pre-computed and stored in UE's memory (in which case the size of such pre-computed codebook would appear to be only slightly less than the size of the original coarse codebook).
Another specific prior art approach is proposed in 3GPP, and refers to the so called hierarchical codebook. In this approach a large amount of standardized codewords are sorted to subgroups based on their closeness to each other. In this way, the codebook has a layered structure. While the layering may simplify the UE's search for the best codeword, still there is a very large codebook which has to be standardized.